1. Field of the Invention
The present invention relates to a LED (light emission diode) driver, and more particularly to an offline control circuit to control the maximum voltage and the maximum current of the LEDs.
2. Description of Related Art
The LED driver is utilized to control the brightness of the LED in accordance with its characteristic. The LED driver is also utilized to control the current that flow through the LED. A higher current increases intensity of the bright of the LED, but decreases the life of the LED. FIG. 1 shows a traditional offline circuit of the LED driver. The output voltage VO of the LED driver is adjusted to provide a current ILED through a resistor 79 to LEDs 71 to 75. The current ILED is shown as,
                              I          LED                =                                                                              V                  O                                -                                  V                                      F                    ⁢                                                                                  ⁢                    71                                                  -                            ...                        -                          V                              F                ⁢                                                                  ⁢                75                                                          R            79                                              (        1        )            
wherein the VF71 to VF75 are the forward voltage of the LEDs 71 to 75 respectively.
The drawback of the LED driver shown in FIG. 1 is the variation of the current ILED. The current ILED is changed in response to the change of the forward voltage of VF71 to VF75. The forward voltages of VF71 to VF75 are not the constant due to the variation of the production and operating temperature. Hence, the maximum voltage and the maximum current of the LEDs 71 to 75, 81 to 85 may overload and decrease the life of the LEDs 71 to 75, 81 to 85.